Fin metal-oxide-semiconductor field effect transistor (Fin-MOSFET) is an emerging technology which provides solutions to metal-oxide-semiconductor field effect transistor (MOSFET) scaling problems at, and below, the 22 nm node. FinMOSFET structures include fin field effect transistors (finFETs) which include at least one narrow semiconductor fin gated on at least two opposing sides of each of the at least one semiconductor fin. FinFET structures may be formed on a semiconductor-on-insulator (SOI) substrate, because of the low source/drain diffusion to substrate capacitance and ease of electrical isolation by shallow trench isolation structures.
However, finFETs fabricated on an SOI substrate suffer from floating body effects, depending on fin thickness, as is well-known for conventional planar MOSFETs. The body of a finFET on an SOI substrate stores charge which is a function of the history of the device, hence becoming a floating body. As such, floating body finFETs experience threshold voltages which are difficult to anticipate and control, and which vary in time. The body charge storage effects result in dynamic sub-threshold voltage (sub-Vt) leakage and threshold voltage (Vt) mismatch among geometrically identical adjacent devices. Floating body effects in finFETs are particularly a concern in static random access memory (sRAM) cells, where Vt matching is extremely important as operating voltages continue to be scaled down. The floating body effects also pose leakage problems for pass gate devices. Further, one of the key concerns of floating body devices is the output conductance instability, a very important factor for analog circuit applications. In view of the above stated problems with finFETs fabricated on SOI substrates, it is desirable to eliminate body effects by building finFETs incorporating body contacts. In addition to this, having a body contact enables devices with multiple threshold voltages by controlling the body voltage.
Methods exist in the prior art for fabricating body-contacted finFETs. However, the prior art designs feature limitations that limit their application to finFETs with only a single fin. For example, U.S. Patent Application Publication No. US 2009/001464 A1 provides for a single-fin finFET with a body contact on the top surface of the fin, formed through the gate. Adapting this method for a multi-fin finFET would at least require forming a separate individual contact to each fin, greatly increasing process complexity, and is potentially impossible due to insufficient space to form multiple body-contacts. U.S. Pat. No. 7,485,520 provides for a single-fin finFET design, where a body contact is formed by removing material from a lower portion of a fin which rests on an adjacent semiconductor substrate, replacing the removed material with an insulating material to isolate the fin, and then forming a contact to the adjacent semiconductor substrate. The complexity of this process would be further increased if adapted to multi-fin designs, where the proximity of adjacent fins would reduce the efficacy of processes to add or remove material from lower portions of the fins. Therefore, a new method of forming body contacts for multi-fin finFETs is desirable.